Morphology and developmental competence of bovine oocytes relative to follicular status

Abstract

In cattle, follicle dimension has been used as the main criterion for selection of oocytes for in vitro embryo production. However, follicles with similar diameters may be in very different physiologic phases. The aim of this study was to investigate whether morphology and developmental competence of cumulus–oocyte complexes (COCs) are related to the phase of development of the follicle, and presence of the corpus luteum (CL) or the dominant follicle in the ovary from which the COCs were collected. Cows ( n=143) were given a luteolytic dose of PGF 2α and 8 days later underwent transvaginal ultrasound guided ablation of follicles ≥4 mm to induce emergence of a new follicular wave. Cows ( n=10–20 per replicate) were slaughtered on Day 2, 3, 5 or 7 (Day 0=follicular wave emergence), equivalent to the growing, early static, late static, and regressing phases of subordinate follicle development. COCs were collected from subordinate follicles ≥3 mm, were classified as denuded, degenerated or healthy, and underwent IVM–IVF–IVC. The proportion of oocytes that developed to the blastocyst stage was higher ( P<0.05) in those collected on Day 5 after wave emergence (23%) than on Day 2 (12%), 3 (13%) or 7 (16%). Data did not support the hypothesis of a local effect of the CL or dominant follicle. We conclude that a positive relationship exists between early follicular regression and oocyte competence. Moreover, morphologic characteristics of oocyte quality used in this study were not predictive in identifying competent oocytes.